Liquid crystal displays (LCDs) are commonly used in devices such as portable televisions, portable computers, control displays, and cellular phones to display information to a user. LCDs act in effect as a light valve, i.e., they allow transmission of light in one state, block the transmission of light in a second state, and some include several intermediate stages for partial transmission. When used as a high resolution information display, as in one application of the present invention, LCDs are typically arranged in a matrix configuration with independently controlled pixels (the smallest segment of the display). Each individual pixel is signaled to selectively transmit or block light from a backlight (transmission mode), from a reflector (reflective mode), or from a combination of the two (transflective mode).
A LCD pixel can control the transference for different wavelengths of light. For example, an LCD can have pixels that control the amount of transmission of red, green, and blue light independently. In some LCDs, voltages are applied to different portions of a pixel to control light passing through several portions of dyed glass. In other LCDs, different colors are projected onto the pixel sequentially in time. If the voltage is also changed sequentially in time, different intensities of different colors of light result. By quickly changing the wavelength of light to which the pixel is exposed an observer will see the combination of colors rather than sequential discrete colors. Several monochrome LCDs can also result in a color display. For example, a monochrome red LCD can project its image onto a screen. If a monochrome green and monochrome blue LCD are projected in alignment with the red, the combination will be full color.
The monochrome resolution of an LCD can be defined by the number of different levels of light transmission that each pixel can perform in response to a control signal. A second level is different from a first level when the user can tell the difference between the two. An LCD with greater monochrome resolution will look clearer to the user.
LCDs are actuated pixel-by-pixel, either one at a time or several simultaneously. A voltage is applied to each pixel and the liquid crystal responds to the voltage by transmitting a corresponding amount of light. In some LCDs an increase in the actuation voltage decreases transmission, while in others it increases transmission. When multiple colors are involved for each pixel, multiple voltages are applied to the pixel at different positions or times depending upon the LCD. Each voltage controls the transmission of a particular color. For example, one pixel can be actuated to allow only blue light to be transmitted while another allows only green. A greater number of different light levels available for each color results in a much greater number of possible combination colors.
Converting a complex digital signal that represents an image or video into voltages to be applied to the pixels of an LCD involves circuitry that can limit the monochrome resolution. The signals necessary to drive a single color of an LCD are both digital and analog. It is digital in that each pixel requires a separate selection signal, but it is analog in that an actual voltage is applied to the pixel to determine light transmission.
Each pixel in the core array of the LCD is addressed by both a column (vertical) driver and a row (horizontal) driver. The column driver turns on an analog switch that connects an analog voltage representative of the video input (control voltage necessary for the desired liquid crystal twist) to the column, and the row driver turns on a second analog switch that connects the column to the desired pixel.
The video inputs to the LCD are analog signals centered around a center reference voltage of typically from about 7.5 to 8.0 volts. This center reference voltage is not a supply or signal from anywhere, rather it is a mathematical entity. This center reference voltage is called “VCOM” and connects to the LCD cover glass electrode which is a transparent conductive coating on the inside face (liquid crystal side) of the cover glass. This transparent conductive coating is typically Indium Tin Oxide (ITO).
One frame of video pixels are run at voltages above the center reference voltage (positive inversion) and for the next frame the video pixels are run at voltages below the center reference voltage (negative inversion). This alternating between positive and negative inversions results in a zero net DC bias at each pixel. This removes the “image sticking” phenomena.
Writing video voltage values to each pixel in, for example, an 800×600 (SVGA) frame takes about 2 milliseconds using 8 analog channels in parallel operation, with each analog channel given about 25 nanoseconds to apply the appropriate video voltage value to each of its set of pixels of the SVGA frame. Unfortunately, the liquid crystal material itself takes about 3 to 4 milliseconds to settle to within one percent of its final reflectivity. That leaves very little time to flash the light source (for example: light emitting diodes—LED) for the illumination step. For example, using a three color frame image at 80 Hz, each of the color (red-green-blue) frames at 240 Hz, allows only 4.2 milliseconds per frame. Considering the requirements imposed by frame inversion, and the problem of color-breakup with color-sequential images, 80 Hz is about the slowest rate at which to present images. With increased resolution of present and future LCD display video images, a faster and more accurate way of writing pixels is desired